Digital images can be stored on servers and distributed over a telecommunication network. Images can also be stored on physical media using for example a CD-ROM. If the image is large it might take some time to transmit the image. In order to speed up the transmission the client might select a region of the image that are more vital and then only receive the region. This is known as Region of Interest (ROI) coding.
In today's client-server systems, if a client wants to access an image from a server, a connection is established between the user program at the client side and the server. The user program then sends a request to get the desired image. The server reacts by fetching the file and starts the transmission. The user program can at any time send a new request to the server, e.g., a request for parts of a file. When the transmission is finished the connection is closed. Such a system is described in Fielding, et al, Hypertext Transfer Protocol—HTTP/1.1 rev 05, HTTP Working Group, INTERNET-DRAFT, Sep. 11, 1998 (Work in progress).
However, there is a problem with most of the still image compression techniques, like e.g. JPEG, in that they produce a bit stream being one inseparable coding unit. Thus, if a Region of Interest is selected the lack of independently decodable units forces the server to perform a complete decoding followed by a new encoding of the whole bit stream. Depending on the server software it sometimes even has to re-load the image from the storage media. The drawback with this is that a time consuming and computational complex scheme is required, which will put high demands on the computational power of the server.
Furthermore, in the emerging still image standard JPEG2000 independent entropy coding of so called coding units (CU) is used. A coding unit can for example be, a subband in the transform domain, in the case of i.e. wavelet transform), a part of a subband, such as bit plane or block of certain size (example 16×16), or a bit plane for a region within a subband.